Ordered structures in homogeneous magnetic fluid thin films and method of preparation

ABSTRACT

Methods for preparing homogeneous magnetic fluid compositions capable of forming ordered one dimensional structures or two dimensional lattices in response to externally applied magnetic fields are disclosed. The compositions are prepared using improved co-precipitation methods in which the steps of the procedure have been tuned to reduce sample heterogeneity. Fe 3  O 4  or MnFe 2  O 4  particles are coated with a surfactant and dispersed in a continuous carrier phase to form these homogeneous magnetic fluid compositions. The ability of these compositions to generate ordered structures can be tested by holding a magnet near a thin film of the compositions and observing the formation of colors in the region near the magnet. Methods for controlling the characteristic spacing of the ordered structures formed by the composition also are disclosed. Relevant parameters include the thickness of the film, the strength and orientation of the externally applied magnetic field, the rate of change of field strength, the volume fraction of the magnetic particles dispersed in the continuous phase, and the temperature of the homogeneous magnetic fluid. The homogeneous magnetic fluid composition is useful for the manufacture of liquid crystal devices. The devices take advantage of the serendipitous fact that the spacings in the material are on the order of the wavelength of visible light. A variety of magnetic-optical devices can be constructed that use the ordered structures to diffract, reflect, and polarize light in a controlled and predictable manner. These devices include color displays, monochromatic light switches, and tunable wavelength filters.

RELATED APPLICATION DATA

The present application is a Continuation-In-Part of application Ser.No. 08/835,107, filed Apr. 4, 1997, titled "Ordered Structures inHomogeneous Magnetic Fluid Thin Films and Method of Preparation," byHerng-Er Horng and Chin-Yih Rex Hong.

FIELD OF THE INVENTION

The present invention comprises methods for producing homogeneousmagnetic fluids capable of forming ordered crystalline structures. Theinvention also comprises methods for generating ordered structures inthin films of such fluids under the influence of externally appliedmagnetic fields, methods for controlling the structures generated inthese films, and magnetic-optical devices based on these orderedstructures. These devices include color displays, monochromatic lightswitches, and tunable wavelength filters.

BACKGROUND

Ferrofluids are a type of magnetic fluid that typically consist ofcolloidal magnetic particles such as magnetite or manganese-zincferrites, dispersed with the aid of surfactants in a continuous carrierphase. The average diameter of the dispersed magnetic particles rangesbetween 5-10 nm. Each particle has a constant magnetic dipole momentproportional to its size that can align with an external magnetic field.

Ferrofluids experience body forces in homogeneous magnetic fields, thatallow their position to be manipulated, and thus enable the constructionof devices such as rotary seals, bearings, and related mechanicaldevices. Ferrofluids also have been used to construct display devicessuch as those disclosed in U.S. Pat. Nos. 3,648,269 and 3,972,595, thatuse a magnetic field to capture an opaque magnetic fluid in apredetermined optical pattern. These types of devices usually operate byhaving an opaque magnetic fluid displace a transparent fluid and therebyproduce optical contrast. Such display devices, however, do not generateordered crystalline structures in the magnetic fluid, and are incapableof generating anything other than a monochromatic image.

Two general methods for producing ferrofluids have been used in theprior art. The first method reduces a magnetic powder to a colloidalparticle size by ball-mill grinding in the presence of a liquid carrierand a grinding aid which also serves as a dispersing agent. Thisapproach is exemplified in U.S. Pat. Nos. 3,215,572 and 3,917,538. Thesecond approach is a chemical precipitation technique as exemplified inU.S. Pat. No. 4,019,994. Both of these techniques suffer from thedisadvantage that there is heterogeneity in the size distribution of theresulting magnetic particles, the composition of these particles, and/orthe interaction forces between the particles. This heterogeneity mayproduce deleterious effects on the ability of a ferrofluid to formordered structures under the influence of a magnetic field.

Pattern forming systems of magnetic fluid films under the influence ofexternal magnetic fields have recently attracted much interest. Forthese studies, a variety of different types of magnetic fluids have beenused. For example, the aggregation process and one-dimensional patternsformed in suspensions of latex or polystyrene particles loaded with ironoxide grains under the influence of parallel fields have been studied byM. Fermigier and A. P. Gast, J. Colloidal Interface Sci. 154, 522(1992), and D. Wirtz and M. Fermigier, Phys. Rev. Lett. 72, 2294 (1994).Quasi two dimensional periodic lattices have been reported to be formedin a phase separated magnetic fluid thin film under the influence of aperpendicular magnetic field. Wang et al., Phys. Rev. Lett. 72, 1929(1994). FIG. 1 of this paper, however, shows that the resultingstructure is disordered. Other investigators have generated more highlyordered two dimensional lattices in thin films of magnetic fluidemulsions or magnetic fluids containing non-magnetic spheres usingperpendicular magnetic fields. However, these lattices tend to solidifyand therefore are not suitable for applications requiring rapidinterconversion between crystalline and amorphous states. See, e.g., Liuet al., Phys. Rev. Lett. 74, 2828 (1995), Skjeltorp, Phys. Rev. Lett.2306 (1983). Thus there is a recognized need in the art for ferrofluidiccompositions that could be used to generate liquid-crystal devices thatcould be switched by small magnetic fields. See, e.g., da Silva andNeto, Phys. Rev. E. 48, 4483 (1993).

If a ferrofluid composition capable of reversibly forming ordered onedimensional structures or crystalline two dimensional lattices in a thinfilm under the influence of an external magnetic field could bemanufactured, it would be useful for constructing a variety of new anduseful liquid-crystal magneto-optical devices. For these reasons, amethod is needed for generating homogeneous ferrofluidic compositionscapable of reversibly forming ordered one dimensional structures orcrystalline two dimensional lattices in a thin film under the influenceof an external magnetic field. Also needed is a simple method fordetermining whether a thin film of a ferrofluidic composition is capableof generating well-ordered one dimensional structures or two dimensionallattices under the influence of external magnetic fields. Finally, itwould be desirable to generate magneto-optical devices based on theordered structures created in thin films of ferrofluidic compositions inresponse to external magnetic fields. Because the utility of suchdevices would be enhanced by developing methods for controlling theordered structures formed in magnetic thin films of ferrofluids underthe influence of external magnetic fields, methods for controlling theordered structures so formed also are needed.

SUMMARY OF THE INVENTION

The present invention is directed to methods for generating homogeneousferrofluidic compositions that are capable of forming ordered structureswhen a thin film of the fluid is subjected to an external magneticfield, as well as compositions synthesized according to this method. Themethod is based on an optimized co-precipitation technique. Theinvention also provides for methods of generating ordered onedimensional structures or two dimensional lattices in thin films ofthese ferrofluidic compositions in response to externally appliedmagnetic fields, as well as methods for determining the ability of ahomogeneous magnetic field to form ordered structures. The inventionalso is directed to the ordered arrays formed in thin films of thehomogeneous ferrofluidic compositions upon exposure to an externalmagnetic field. Also provided are methods for controlling thecharacteristic spacings of the one dimensional structures or twodimensional lattices by varying parameters such as the strength of theapplied magnetic field, the orientation of the field to the film, therate of change of magnetic field strength, the film thickness, theconcentration of magnetic particles in the ferrofluidic composition, orthe temperature of the composition. Further, the invention provides forliquid-crystal magnetic-optical devices based on ordered structurescreated in thin films of ferrofluids and the ability to control thespacings of these structures. These devices include: a light diffractioncolor display, a monochromatic light diffraction switch that can beturned on or off, a tunable light diffraction wavelength filter, asecond type of light diffraction color display that combines thetechnologies of the first light diffraction color display and themonochromatic light diffraction switch, and a light double refractioncolor display comprising the magnetic fluid thin film of the presentinvention and polarizers.

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed in color.Copies of this patent with color drawing(s) will be provided by thePatent and Trademark Office upon request and payment of the necessaryfee.

FIG. 1 is a flow chart diagram of the steps for preparation ofhomogeneous ferrofluids capable of forming ordered one dimensionalstructures or two dimensional lattices when a thin film of the fluid issubjected to an external magnetic field. FIG. 1a is for preparation ofan Fe₃ O₄ fluid. FIG. 1b is for an MnFe₂ O₄ fluid.

FIG. 2 illustrates a setup for measuring the properties of ferrofluidicthin films under externally applied magnetic fields.

FIGS. 3A-D illustrates two-dimensional hexagonal arrays with particlecolumns occupying lattice vertices generated in an Fe₃ O₄ fluidhomogeneous ferrofluidic thin film in response to an externally appliedmagnetic field oriented perpendicularly to the plane of the film.

FIGS. 4A-D shows two-dimensional hexagonal arrays formed in Fe₃ O₄ fluidfilms with different thicknesses in response to a perpendicular, 100 Oemagnetic field.

FIG. 5 is a graph showing the relation of the distance between Fe₃ O₄particle columns in two-dimensional hexagonal arrays to magnetic fieldstrength and film thickness.

FIG. 6 is a graph showing the relation of the distance between Fe₃ O₄particle columns in two-dimensional hexagonal arrays to the magneticfield strength and the rate of change of magnetic field strength.

FIG. 7 is a graph relating the distance between particle columns intwo-dimensional hexagonal arrays to the magnetic field strength and thevolume fraction ratio between the magnetic particle and liquid carriercomponents of the Fe₃ O₄ ferrofluid.

FIGS. 8A-D illustrates the relationship between the periodic spacing ofparticle chains formed in a homogeneous Fe₃ O₄ ferrofluidic thin filmand the strength of an external magnetic field that is parallel to theplane of the film.

FIGS. 9A-D illustrates the relationship between the periodic spacing ofparticle chains formed in a homogeneous Fe₃ O₄ ferrofluidic thin filmexposed to a parallel external magnetic field as a function of filmthickness.

FIGS. 10A-D illustrates two-dimensional hexagonal arrays with particlecolumns occupying lattice vertices generated in an MnFe₂ O₄ fluidhomogeneous ferrofluidic thin film in response to an externally appliedmagnetic field oriented perpendicularly to the plane of the film.

FIGS. 11A-D shows two-dimensional hexagonal arrays formed in MnFe₂ O₄fluid films with different thicknesses in response to a perpendicular,75 Oe magnetic field.

FIG. 12 is a graph showing the relation of the distance between MnFe₂ O₄particle columns in two-dimensional hexagonal arrays to magnetic fieldstrength and film thickness.

FIG. 13 is a graph showing the relation of the distance between MnFe₂ O₄particle columns in two-dimensional hexagonal arrays to the magneticfield strength and the rate of change of magnetic field strength.

FIG. 14 is a graph relating the distance between particle columns intwo-dimensional hexagonal arrays to the magnetic field strength and thevolume fraction ratio between the magnetic particle and liquid carriercomponents of the MnFe₂ O₄ ferrofluid.

FIGS. 15A-D illustrates the relationship between the periodic spacing ofparticle chains formed in a homogeneous MnFe₂ O₄ ferrofluidic thin filmand the strength of an external magnetic field that is parallel to theplane of the film.

FIGS. 16A-D illustrates the relationship between the periodic spacing ofparticle chains formed in a homogeneous Fe₃ O₄ ferrofluidic thin filmexposed to a parallel external magnetic field as a function of filmthickness.

FIG. 17 illustrates a setup used for demonstrating light diffraction anddouble refraction phenomena generated by ordered structures inhomogeneous ferrofluidic thin films.

FIG. 18 shows a spectrum of colors produced by a magneto-optical devicein which the thickness of the homogeneous Fe₃ O₄ ferrofluidic thin filmvaries from about 2 to 10 μm. ##EQU1##

FIGS. 19A-F shows different colors produced by a magneto-optical devicecomprising a homogeneous Fe₃ O₄ ferrofluidic thin film as the externallyapplied magnetic field strength is varied.

FIG. 20 illustrates the cross-section of a homogeneous ferrofluidic thinfilm for a first type of light diffraction display device.

FIG. 21 illustrates the design of an individual pixel element comprisinga homogeneous ferrofluidic thin film, a means for generating a magneticfield, and a means for controlling the strength of the field.

FIG. 22 illustrates a cross section of a homogeneous ferrofluidic thinfilm for a double refraction display device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The chemical synthesis of magnetite (Fe₃ O₄) by co-precipitation ofFeSO₄ and FeCl₃ in the presence of NaOH is based on a reaction proposedby W. C. Elmore in 1938. This co-precipitation reaction has been used togenerate ferrofluids (also referred to as "magnetic fluids") in whichthe magnetite particles are coated with a surfactant and dispersed in acontinuous phase (i.e., the particles are dispersed in a liquid that isnot an emulsion). See, e.g., Fertman, V. E., "Magnetic Fluids GuideBook: Properties and Application," Hemisphere Publishing Corporation,1989, ISBN-0-89116-956-3 at page 14. While such materials have provedextremely useful for the construction of various mechanical and displaydevices, they have not been amenable to forming ordered structures inthin films. Ordered structures are regular, periodic arrays of objects,that interact with electromagnetic radiation (e.g., visible light) togenerate physical phenomena such as diffraction or polarization. Thesestructures may be ordered in two dimensions (e.g., x and y), or onedimension (e.g., x). The former structures are sometimes also referredto as lattices, crystalline arrays, or 2-dimensional crystals. Bycarefully tuning the parameters of the co-precipitation reaction andsubsequent coating and dispersing steps, we have synthesized improvedferrofluidic compositions capable of reversibly forming orderedstructures in thin films under the influence of external magneticfields.

While not wishing to be bound by any particular theory, it seems likelythat improvements in the homogeneity of particle size distributionand/or interaction forces between the particles might be responsible forthe ability of these ferrofluidic compositions to form orderedstructures. Improvements in interaction force homogeneity in theferrofluidic compositions of the instant invention may reflect reducedcontamination of the compositions by Fe₂ O₃ and/or water.

According to the methods of the present invention, a composite materialcomprising ultra-fine magnetic particles uniformly dispersed in acontinuous liquid phase is prepared by co-precipitation techniques whosecontrolling parameters were carefully tuned. In a first embodiment themagnetic particles are Fe₃ O₄ (magnetite) and result from a chemicalreaction between a mixture of FeSO₄ and FeCl₃ and alkali such as NaOH,Fe(OH)₂, or Fe(OH)₃. In a second embodiment the magnetic particles areMnFe₂ O₄ and result from a chemical reaction between a mixture of MnSO₄and FeCl₃ and alkali such as NaOH, Fe(OH)₂, or Fe(OH)₃. The particlesare coated with a layer of surfactant to prevent agglomeration, and aredispersed throughout a continuous liquid carrier phase to form ahomogeneous magnetic fluid. FIG. 1 shows a flow chart diagram of thesteps used to prepare a homogeneous magnetic fluid according to thepresent invention.

The general procedure used in the first embodiment involves making anaqueous solution of FeSO₄ and FeCl₃. The temperature of the solution ismaintained at 80° C. and is continuously stirred while a sufficientamount of a hydroxide containing base solution such as NaOH, Fe(OH)₂, orFe(OH)₃ is rapidly added to keep the pH of the solution betweenapproximately 11 and 11.5. It is important that not more than about 2minutes elapse between the start of the base addition, and theattainment of the target pH value. The co-precipitation of Fe₃ O₄ occursover about a 20 minute time period. The formula for this reaction is asfollows:

    8NaOH+FeSO.sub.4 +2FeCl.sub.3 →Fe.sub.3 O.sub.4 ↓+Na.sub.2 SO.sub.4 +6NaCl+4H.sub.2 O

After about 20 minutes, a surfactant such as oleic acid is added to thesolution out of which the Fe₃ O₄ has precipitated. This serves to coatthe Fe₃ O₄ particles. If the surfactant added is oleic acid, the pHvalue drops substantially at first, and an additional amount of basesolution is added to keep the pH at a preferred range from about 9.5 toabout 10 during the coating process. During the coating process, thetemperature of the reaction is maintained at 80° C. This process takesaround 30 minutes. At the end of this step, the reaction mix separatesinto three phases. Prior to proceeding to the next step, the upper layeris removed and discarded, and the middle and bottom layers are retainedfor use in the next step of the process. The formulae of the chemicalreactions that occur during the coating process are as follows whenoleic acid is used as the surfactant and NaOH is used as the base:

    1. CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COOH+Na.sup.+ OH.sup.- →CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ +H.sub.2 O

    2. Fe.sub.3 O.sub.4 +CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ →Fe.sub.3 O.sub.4 ·[CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ ]

After the coating process has completed (around 30 minutes), anacidification step is carried out to protonate the carboxylate group andthereby replace the Na⁺ counterion with a proton. This is achieved byadding a sufficient amount of an acid such as HCl to the reaction mix asit is stirred to bring to the pH of the mixture down to a range of fromaround 0 to around 1. This step is carried out at room temperature (fromabout 20° C. to about 25° C.). The mix is stirred for approximately 20minutes. During this time, magnetic particles coated with surfactantbegin to coagulate. At the end of approximately 20 minutes, the mixseparates into two phases. The top phase is removed, and theacidification step may be repeated as before an additional two or threetimes. At the end of each acidification step cycle, the top phase isremoved prior to repeating this step. When the top phase no longercontains dark particulate material, the next step may be performed. Theformulae of the chemical reactions occurring during this replacementstep is as follows when HCl is used as the acid:

    Fe.sub.3 O.sub.4 ·[CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ ]+H.sup.+ Cl.sup.- →Fe.sub.3 O.sub.4 ·[CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- H.sup.+ ]+Na.sup.+ Cl.sup.-

The next step is decantation. During this step, de-ionized water isadded to remove remaining counter ions such as HCl and NaCl from thesurfactant-coated Fe₃ O₄ product. A sufficient amount of de-ionizedwater at 65° C. is added to the coated Fe₃ O₄ to bring the pH of thesuspension to a value between around 4.7 to 5.0. The suspension isstirred as the water is added. After a sufficient amount of de-ionizedwater has been added, the stirring is stopped and the suspension isallowed to settle. The water is decanted away from the settled Fe₃ O₄and the product is washed.

Washing is achieved by adding a liquid used as a carrier (e.g.,kerosene) to the settled Fe₃ O₄ in a ratio of approximately 1.1milliliter of kerosene per gram of coated Fe₃ O₄. The two components arestirred until the solid Fe₃ O₄ is completely suspended in the carrier.This suspension is placed in a centrifuge tube and subjected to a short,low-speed spin carried out at room temperature. We have found that a 10minute spin at a relative centrifugal force equivalent to about 500×gworks well when kerosene is used as the carrier. When the sample isremoved from the centrifuge, it will have separated into two phases. Thetop phase is a dark-colored liquid that contains salt residues and largeparticles, while the lower phase is a solid that contains magneticparticles coated with surfactant. The top phase is removed, and thecoated magnetic particles are dehydrated as completely as ispracticable.

We have found that suitable dehydration can be achieved by suspendingthe particles in acetone, pelleting them with a 30 minute centrifugationat 1800×g, removing the acetone, and drying the particles for 8 to 12hours in a 65° C. oven. After the particles have been dehydrated, theyare dispersed in the carrier, and the fluid is subjected to anothershort, low-speed spin in a centrifuge. This spin pellets larger oraggregated particles. The liquid sitting above any pellet that may beformed in this spin is the homogeneous magnetic fluid of the presentinvention. The concentration of magnetic particles in the fluid may beincreased by setting the fluid in a 65° C. oven for 8 to 12 hours toevaporate a portion of the carrier and thereby raise the Fe₃ O₄concentration. The fluid was removed from the oven and aliquots of thishomogeneous magnetic fluid were sealed into glass cells to form Fe₃ O₄magnetic fluid thin films.

The general procedure used in the second embodiment involves making anaqueous solution of MnSO₄ and FeCl₃. The temperature of the solution ismaintained at 80° C. and is continuously stirred while a sufficientamount of a hydroxide containing base solution such as NaOH, Fe(OH)₂, orFe(OH)₃ is rapidly added to keep the pH of the solution betweenapproximately 10 and 11. It is important that not more than about 2minutes elapse between the start of the base addition, and theattainment of the target pH value. The co-precipitation of MnFe₂ O₄occurs over about a 60 minute time period. The formula for this reactionis as follows:

    8NaOH+MnSO.sub.4 +2FeCl.sub.3 →MnFe.sub.2 O.sub.4 ↓+Na.sub.2 SO.sub.4 +6NaCl+4H.sub.2 O

After about 60 minutes, a surfactant such as oleic acid is added to thesolution out of which the MnFe₂ O₄ has precipitated. This serves to coatthe MnFe₂ O₄ particles. If the surfactant added is oleic acid, the pHvalue drops substantially at first, and an additional amount of basesolution is added to keep the pH at a preferred range from about 9.5 toabout 10 during the coating process. During the coating process, thetemperature of the reaction is maintained at 95° C. This process takesaround 50 minutes. At the end of this step, the reaction mix separatesinto three phases. Prior to proceeding to the next step, the upper layeris removed and discarded, and the middle and bottom layers are retainedfor use in the next step of the process. The formulae of the chemicalreactions that occur during the coating process are as follows whenoleic acid is used as the surfactant and NaOH is used as the base:

    1. CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COOH+Na.sup.+ OH.sup.- →CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ +H.sub.2 O

    2. MnFe.sub.2 O.sub.4 +CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ →MnFe.sub.2 O.sub.4 ·[CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ ]

After the coating process has completed (around 50 minutes), anacidification step is carried out to protonate the carboxylate group andthereby replace the Na⁺ counterion with a proton. This is achieved byadding a sufficient amount of an acid such as HCl to the reaction mix asit is stirred to bring to the pH of the mixture down to a range ofaround 1. This step is carried out at room temperature (from about 20°C. to about 25° C.). The mix is stirred for approximately 20 minutes.During this time, magnetic particles coated with surfactant begin tocoagulate. At the end of approximately 20 minutes, the mix separatesinto two phases. The top phase is removed, and the acidification stepmay be repeated as before an additional two or three times. At the endof each acidification step cycle, the top phase is removed prior torepeating this step. When the top phase no longer contains darkparticulate material, the next step may be performed. The formulae ofthe chemical reactions occurring during this replacement step is asfollows when HCl is used as the acid:

    MnFe.sub.2 O.sub.4 ·[CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ ]+H.sup.+ Cl.sup.- →MnFe.sub.2 O.sub.4 ·[CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- H.sup.+ ]+Na.sup.+ Cl.sup.-

The next step is decantation. During this step, de-ionized water isadded to remove remaining counter ions such as HCl and NaCl from thesurfactant-coated MnFe₂ O₄ product. A sufficient amount of de-ionizedwater at 65° C. is added to the coated MnFe₂ O₄ to bring the pH of thesuspension to a value between around 4.7 to 5.0. The suspension isstirred as the water is added. After a sufficient amount of de-ionizedwater has been added, the stirring is stopped and the suspension isallowed to settle. The water is decanted away from the settled MnFe₂ O₄and the product is washed.

Washing is achieved by adding a liquid used as a carrier (e.g.,kerosene) to the settled MnFe₂ O₄ in a ratio of approximately 1.1milliliter of kerosene per gram of coated MnFe₂ O₄. The two componentsare stirred until the solid MnFe₂ O₄ is completely suspended in thecarrier. This suspension is placed in a centrifuge tube and subjected toa short, low-speed spin carried out at room temperature. We have foundthat a 10 minute spin at a relative centrifugal force equivalent toabout 500×g works well when kerosene is used as the carrier. When thesample is removed from the centrifuge, it will have separated into twophases. The top phase is a dark-colored liquid that contains saltresidues and large particles, while the lower phase is a solid thatcontains magnetic particles coated with surfactant. The top phase isremoved, and the coated magnetic particles are dehydrated as completelyas is practicable.

We have found that suitable dehydration can be achieved by suspendingthe particles in ethyl alcohol, pelleting them with a 15 minutecentrifugation at around 2800×g, removing the ethyl alcohol, and dryingthe particles for 10 to 14 hours in a 65° C. oven. After the particleshave been dehydrated, they are dispersed in kerosene using akerosene-to-particle ratio of 1.7 ml/g, and the fluid is centrifuged ataround 2800×g for about 10 minutes. This spin pellets larger oraggregated particles. The liquid sitting above any pellet that may beformed in this spin is the homogeneous magnetic fluid of the presentinvention. The concentration of magnetic particles in the fluid may beincreased by setting the fluid in a 65° C. oven for approximately 14hours to evaporate a portion of the carrier and thereby raise the MnFe₂O₄ concentration. Aliquots of this homogeneous magnetic fluid weresealed into glass cells to form MnFe₂ O₄ magnetic fluid thin films.

In addition to the kerosene and oleic acid combination described above,other pairs of carriers and surfactants may be used to generate thecompositions of the present invention that are capable of formingordered structures in thin films. Table 1 sets out representativecombinations. In this table, any of the carriers listed in a cell may beused with any of the surfactants listed in the cell in the same row ofthe table.

                                      TABLE 1                                     __________________________________________________________________________    Carrier/Surfactant Combinations Useful for Generating Homogeneous             Magnetic Fluids                                                               Carrier     Surfactant                                                        __________________________________________________________________________      kerosene  1.                                                                              oleic acid                                                        2. cyclohexane (C.sub.6 H.sub.12) 2. lineoleic acid                           3. n-octane (C.sub.8 H.sub.18) 3. olive oil, a mixture of:                    4. n-dodecane (C.sub.12 H.sub.26)  ˜9% CH.sub.3 (CH.sub.2).sub.14                   COOH,                                                             5. n-tetradecane (C.sub.14 H.sub.30)  ˜2% CH.sub.3 (CH.sub.2).sub.                  16 COOH,                                                          6. n-hexadecane (C.sub.16 H.sub.34)  ˜80% oleic acid,                   7. n-octadecane (C.sub.18 H.sub.38)  ˜10% CH.sub.3 (CH.sub.2).sub.                  4 CH═CH--CH.sub.2 --CH═CH--(CH.sub.2).sub.7 --COOH                     8. n-eicosane (C.sub.20 H.sub.42) 4. [R(COO)].sub.2 Zn                       (where R = CH.sub.3 (CH.sub.2).sub.n,                                and 4 ≦ n ≦ 5)                                                 5. eurcic acid                                                            perfluoroeicosane (C.sub.20 F.sub.42)                                                     1.                                                                              oleic acid                                                         2. perfluoropolyether acid                                                     CF.sub.3 CF.sub.2 [CF.sub.2 OCF(CF.sub.3)].sub.5 COOH                       gas oil, C.sub.12 and above 1. oleic acid                                     hydrocarbon 2. olive oil                                                       3. [R(COO)].sub.2 Zn (where R = CH.sub.3 (CH.sub.2).sub.n,                     and 4 ≦ n ≦ 5)                                                perfluoro kerosene 1. perfluoropolyether acid                                  2. oleic acid                                                                 3. olive oil                                                               2-methoxyethyl ether                                                                      mixture of R--O--R' (where R or R' =                                 CH.sub.3 (CH.sub.2).sub.n, and 4 ≦ n ≦ 5) and R need not        equal R'                                                                   __________________________________________________________________________

Characterization of the Homogeneous Magnetic Fluids

Based on the procedures outlined above, homogeneous magnetic fluids areprepared. X-ray diffraction patterns of the samples can be used toverify the single phase fcc spinel structure expected for an Fe₃ O₄ orMnFe₂ O₄ sample. The X-ray diffraction data can be compared to standardsobtained from the International Center for Diffraction Data compiled bythe Joint Committee on Powder Diffraction Standards. The magnetizationof the sample is measured using a vibration sample magnetometer such asa VSM Controller Model 4500 available from EG&G Princeton AppliedResearch. The particle size of the sample is determined from themagnetization, applied field data (M-H data) by fitting it to theLangevin function L(α)=M/M_(s) =(coth α-1/α), where α=M_(s) VH/kT, M isthe magnetization of the sample at an applied magnetic field strength,H, M_(s) is the saturated magnetization of the sample, and V is thevolume of a particle. Thus, because temperature, M, M_(s), k and H areknown, V can be solved for and the radius of the particle determined.The Langevin function assumes: (1) a uniform particle size; and (2)independent particle behavior. In a magnetic fluid, the particle sizemay be described by a normal distribution, and interactions betweenparticles occur because the particles generate magnetic dipoles. Thus,the closer the agreement between the empirical M-H curve and thecalculated Langevin function, the better the assumptions underlying theLangevin function are met. If a magnet is held near homogeneous magneticfluid thin films manufactured according to the methods of the presentinvention, a light color appears in the film and moves as the magnet ismoved due to optical effects created by ordered structures formed inresponse to the magnetic field. Such colors are not seen in EMG 909, acommercially available kerosene-based Fe₃ O₄ ferrofluid obtained fromFerrofluidics Corp. (Nashua, N.H.).

Ordered Structures in Thin Films of Homogeneous Magnetic Fluids

A magnetic fluid synthesized according to the methods outlined above maybe sealed into a number of glass cells with various cell thicknesses toform fluidic thin films. The thin films of the present invention havepreferred thicknesses in the range of from about 1 micron to about 20microns, and more preferably from about 2 microns to about 6 microns (1micron=10⁻⁶ m). In one embodiment of the invention, magnetic fieldsparallel to and perpendicular to the plane of the film may be generatedby Helmholtz coils and by a uniform solenoid, respectively. The magneticfield strength of the coils and solenoid may be related to the currentsupplied to these devices by using a gauss meter to measure the magneticfield. The resulting magnetic fields are uniform, with deviations offield strength in the region of the film less than 1%. To characterizethe ordered structures produced by applied magnetic fields in thin filmsof the homogeneous magnetic fluids, we photographed the films using aZeiss optical microscope, and the time evolution of the formation ofpatterns in the films was recorded using a personal computer through aCCD video camera.

FIG. 2 illustrates a setup useful for measuring properties ofhomogeneous magnetic fluid thin films under externally applied magneticfields. The power supply used for generating the magnetic fields iscomputer controlled and is programmed such that the image data isobtained automatically. The program controlling the data acquisition iswritten such that the field strength and the rate of change of fieldstrength can be adjusted. If desired, a delay time may be programmedprior to capturing image data after the field strength has been changedto ensure the pattern has reached a quasi-steady state.

When a homogeneous magnetic fluid thin film prepared according to themethods of the present invention is subjected to a perpendicularlyapplied magnetic field (i.e., the field direction is normal to the planeof the film), initial disorder quantum columns form. If the fieldstrength is increased so that it exceeds a critical value, H_(h), anequilibrium two-dimensional hexagonal structure forms with particlecolumns occupying lattice vertices. If the field strength is increasedto another critical value, H_(l), the pattern changes from a hexagonalstructure to a labyrinthine pattern. FIG. 3 illustrates this phenomenonin a 6 μm thin film of Fe₃ O₄. In the range of field strength betweenH_(h) and H_(l), the distance between the particle columns is almostlinearly proportional to the inverse of the field strength; the distancebetween the particle columns is on the order of several microns (FIG.5). In contrast, commercially available magnetic fluids only generatedisordered quantum columns under the influence of perpendicularlyapplied magnetic fields.

Other parameters affecting the distance between the particle columnsinclude film thickness, L, (FIGS. 4, 5, 11, and 12), the rate of changeof the field strength, dH/dt, (FIGS. 6 and 13), the magnetic particleconcentration in the fluid (volume fraction ratio; FIGS. 7 and 14), andtemperature, T. The distance between columns is directly proportional tothe magnetic film thickness (FIGS. 5 and 12). An increase in the rate ofchange of field strength (dH/dt) tends to decrease the distance betweencolumns for the same final field strength. This may be due to a boundaryeffect. In a plot of the distance (d) between columns (on the ordinate)as a function of field strength (H) (abscissa), a curve generated usinga larger rate of field strength change will lie below and to the left ofa curve generated using a smaller rate of field strength change. Volumefraction ratio may be adjusted by diluting the magnetic fluid withadditional carrier. Decreasing the volume fraction tends to increase thedistance between columns, when film thickness and rate of field strengthchange are held constant. Thus, a plot of distance as a function offield strength at two different volume fractions shows that the d-Hcurve shifts up and to the right as the volume fraction is reduced. Anincrease in temperature (T) results in a decrease in the magnetizationof the particles, and produces an increase in the distance betweencolumns as all other parameters are held constant.

If a thin film of a homogeneous magnetic fluid of the present inventionis subjected to a parallel magnetic field, the magnetic particles in thethin film agglomerate and form chains parallel to the direction of thefield. As the field strength is increased, these chains tend toaggregate and form coarse, long chains because of their interaction. Aone-dimensional quasi-periodic structure has been observed in thin filmsof homogeneous magnetic fluids of the present invention. The chainsexist in different layers over the thickness of the film. The distancebetween particle chains is inversely proportional to the field strength(FIGS. 8 and 15), and proportional to film thickness (FIGS. 9 and 16).

Magnetic-optical Devices Using Homogeneous Magnetic Fluid Thin Films

The present invention also relates to optical phenomena created whenelectromagnetic waves pass through or are reflected by the controllableordered structures produced in homogeneous magnetic fluid thin films ofthe present invention upon exposure to externally applied magneticfields. To demonstrate these phenomena, the setup illustrated in FIG. 17was used to construct and test magnetic-optical devices. The area of thethin film used was 1 cm×4 cm. Helmholtz coils and a uniform solenoidwere respectively used to generate parallel and perpendicular magneticfields. The resulting fields were uniform with a measured deviation offield strength in the vicinity of the thin film of less than 1%. A whitelight source was used (Intralux 500-1 240 Watt halogen lamp, VOLPIManufacturing, Inc., USA, lamp operated at approximately 25% maximumpower). The light rays were made near parallel by passing them through atelescope. Two optical lenses were used to make the near-parallel lightparallel. An aperture was placed between the two lenses to control thesize of the light beam. The parallel white light was reflected by amirror located beneath the thin film. The angle of the mirror to thelight beam was adjustable by turning the mirror plane, resulting in achange of the incident angle of the light to the film. Photographicimages of the light through the thin film were taken using a CCD camerathat was connected to a computer for data acquisition. In addition, aconventional film camera was sometimes used to obtain images of the thinfilm.

FIG. 18 is a photograph of a drop of a homogeneous Fe₃ O₄ magnetic fluidexposed to an externally applied perpendicular magnetic field. Thethickness of the drop varies because of surface tension effects. Becausethe spacing of the ordered arrays formed in response to the externalmagnetic field vary as a function of film thickness, a spectrum ofcolors is seen when a source of parallel white light is placed below thefilm. The scale bar corresponds to 2 mm.

FIG. 19 is a series of photographic images of a homogeneous Fe₃ O₄magnetic fluid thin film that illustrate diffraction of light by thefilm under the influence of an externally applied perpendicular magneticfield. The scale bar on the figure corresponds to 2 mm. In these images,all the parameters were kept constant, except the current to thesolenoid used to generate the magnetic field. The color of the filmchanges from red to violet as the magnetic field is altered. Theseimages demonstrate that the color of light passing through the thin filmcan be controlled, and that monochromatic light can be obtained from athin film with an area on the order of several square centimeters.

A display device comprising a plurality of pixels, each of whichcomprises a magnetic thin film with an independent electronic circuitfor controlling the magnetic field or temperature experienced by thefilm can therefore be constructed according to the methods of thepresent invention. By properly adjusting the current in each pixel, apolychromatic image may be displayed.

EXAMPLE 1 Preparation of a Homogeneous Fe₃ O₄ Magnetic Fluid Composition

500 mls. of an 8 molar solution of NaOH was made by adding 160 g of NaOH(95% grade, Nihon Shiyaku Industries, Ltd.) to a sufficient amount ofde-ionized water to bring the final volume to 500 mls. A second solutionwas made by mixing 0.1 moles of FeSO₄.7H₂ O (98% grade, Showa Chemicals,Inc.) and 0.2 moles of FeCl₃.6H₂ O (97% grade, Showa Chemicals, Inc.) ina sufficient volume of de-ionized water to bring the final volume to 600mls. A glass stirring bar was used to continuously stir the secondsolution while a sufficient volume of NaOH was added to raise the pH andmaintain it between 11 and 11.5, as Fe₃ O₄ precipitated out of thesolution. The addition of NaOH was completed in under about 2 minutes.During this step, the temperature was held at 80° C. The precipitationprocess took about 20 minutes.

50 mls. of oleic acid (Showa Chemicals, Inc.) was added to the solutioncontaining the Fe₃ O₄ precipitate to coat the particles with oleic acid.At first, the pH value dropped substantially, and an additional volumeof the NaOH solution was added to keep the pH at around 10 during thecoating process. During this procedure, the temperature was maintainedat 80° C. The coating process took approximately 30 minutes. At the endof this step, the solution separated into three phases. The upper phasewas removed and discarded, and the middle and bottom phases wereretained for use in the following step.

A volume of HCl (37.52%, Polin) was added to the retained solutionsufficient to bring the pH down to about 1. This step was carried out atroom temperature. The solution was stirred for about 20 minutes, asmagnetic particles coated with oleic acid began to coagulate. At the endof the 20 minutes, the solution separated into two phases. The topphase, containing a black particulate suspension, was removed, and theentire acidification procedure was repeated as before. Again the topphase was removed and discarded. The black particulate suspension wasnot observed in the top phase formed following the second acidificationstep. In some syntheses, this step may have to be repeated an additionalone or two times, until the black particulate suspension is no longerobserved in the top phase, removing the top phase between repetitions ofthe process.

After the acidification steps were completed, de-ionized water at 65° C.was added to the retained bottom phase in order to remove remaining HCland NaCl. The water was added as the material is stirred. A sufficientvolume of water was added to raise the pH of the material to around 5.The solid material was allowed to settle and the water was decanted.

The solid material was washed by adding 1.1 ml. of kerosene per gram ofsolid. This was stirred until the solid material was completelydispersed in the kerosene, and the solution was placed in a centrifugetube and centrifuged for 10 minutes at around 500×g. This and all othercentrifugation steps were carried out at room temperature. Aftercentrifugation, the sample had separated into two layers. The top layerwas a dark colored liquid containing salt residues and large particles,and the lower layer was a solid phase which contained magnetic particlescoated with oleic acid. The upper phase was removed, and the magneticparticles were dehydrated by suspending the material in acetone,pelleting it by centrifugation for 30 minutes at around 1800×g, removingthe acetone, and drying the magnetic particles in a 65° C. oven for 8 to12 hours. Finally, the dried particles were dispersed into keroseneusing a kerosene-to-particle ratio of 2 ml/g. This was centrifuged againat around 500×g for 10 minutes. The supernatant was removed from thetest tube and placed in a 65° C. oven for approximately 10 hours todrive off a portion of the kerosene and thereby raise the Fe₃ O₄concentration. The fluid was removed from the oven and was used to formordered structures in thin films. Aliquots of this homogeneous magneticfluid were sealed into glass cells to form magnetic fluid thin films.

An X-ray diffraction pattern for the Fe₃ O₄ sample verified the singlephase fcc spinel structure of the sample. The lattice constant wasmeasured to be 8.40 Å. The magnetization of the sample was measuredusing a vibration sample magnetometer, and a saturation magnetizationvalue of 10.58 emu/g was measured for the homogeneous magnetic fluid.The volume fraction of the homogeneous magnetic fluid was calculated asthe ratio of the saturated magnetization of the magnetic fluid to thatof the dry Fe₃ O₄ powder. This ratio was 18.9%.

EXAMPLE 2 Two-dimensional Ordered Structures in Fe₃ O₄ Thin Film as aFunction of Applied Field Strength

The setup illustrated in FIG. 2 was used to examine pattern formation ina thin film of the homogeneous Fe₃ O₄ magnetic fluid thin filmsynthesized in Example 1 in response to an externally applied magneticfield oriented perpendicularly to the plane of the film. In thisexample, the strength of the applied field was varied. FIG. 3 showsimages taken of a 6 μm thick magnetic fluid thin film using a CCD videocamera that demonstrate the evolution of the two-dimensional orderedstructure pattern from disorder quantum columns (FIG. 3a), to an orderedhexagonal structure (FIGS. 3b and 3c), and to a disordered labyrinthinepattern (FIG. 3d). These images illustrate that the distance betweencolumns was roughly inversely proportional to the field strength in therange between two critical strengths, H_(h) and H_(l).

EXAMPLE 3 Two-dimensional Ordered Structures in Fe₃ O₄ Thin Films as aFunction of Film Thickness

In this example, the effect of film thickness on pattern formation wasexamined. The two-dimensional ordered structures in homogeneous Fe₃ O₄magnetic fluid thin films with different thicknesses were investigatedusing the setup illustrated in FIG. 2. During this experiment, allparameters remain unchanged except the thickness of the film, which wasvaried from 10 μm to 2 μm by using glass sample cells having differentcell depths. FIG. 4 provides examples of images of thin films of thehomogeneous magnetic fluid synthesized in Example 1 taken by the CCDcamera using a constant field strength of 100 Oe in which atwo-dimensional hexagonal structure had formed in the films underinvestigation. These images indicate that the distance between columnsis roughly proportional to the thickness of the films over the range offilm thickness examined.

The results obtained in Examples 2 and 3 show that a two-dimensionalhexagonal structure forms in homogeneous Fe₃ O₄ magnetic fluid thinfilms that are subjected to an externally applied perpendicular magneticfield. The distance between columns is closely related to the inverse ofthe field strength and is roughly proportional to film thickness, atleast over the range of thickness shown in Example 3. FIG. 5 plots thedistance between columns as a function of film thickness and magneticfield strength.

EXAMPLE 4 Two-dimensional Ordered Structures in Fe₃ O₄ Thin Films as aFunction of the Rate of Change of Field Strength

The effect of the rate of change of the magnetic field strength wasinvestigated, using rates of 5 Oe/s, 50 Oe/s, 100 Oe/s, and 400 Oe/s.FIG. 6 shows the relationship between column distance as a function offield strength using different rates of field strength change (dH/dt).The figure shows that as the rate is increased, the curves are displaceddownward and to the left. That is, as the rate of field strength changeincreases, the distance between the columns decreases.

EXAMPLE 5 Two-dimensional Ordered Structures in Fe₃ O₄ Thin Films as aFunction of Volume Fraction Ratio

Fe₃ O₄ magnetic fluid samples with varying volume fraction ratiosbetween the magnetic particles and the carrier liquid were madeaccording to the method of Example 1, except in the final dispersingstep, the volume of kerosene added was altered to vary the volumefraction ratio of the fluid. FIG. 7 shows that a decrease in the volumefraction ratio produces a shift in the distance versus field strengthplots toward the upper right. That is, holding all other parametersconstant, a decrease in the volume fraction ratio increases the distancebetween columns.

In the following two examples, the solenoid was replaced by Helmholtzcoils in the setup shown in FIG. 2. As a result, the orientation of themagnetic field applied to the thin film was parallel to the plane of thefilm.

EXAMPLE 6 One-dimensional Ordered Structures in Fe₃ O₄ Thin Films as aFunction of Applied Field Strength

In this example, the homogeneous Fe₃ O₄ magnetic fluid thin film wassubjected to an externally applied magnetic field that was parallel tothe plane of the film. As the field was applied, the magnetic particlesin the film agglomerated and formed chains in the plane of the thin filmoriented along the field direction. These particle chains exist indifferent layers over the thickness of the film. When the field strengthwas increased, the chains became periodic and the distance between thechains decreased proportionately. FIG. 8 shows the effect of varying thefield strength from 100 Oe to 400 Oe on the distance between theperiodic particle chains in the homogeneous Fe₃ O₄ magnetic fluid thinfilm.

EXAMPLE 7 One-dimensional Ordered Structures as a Function of Fe₃ O₄Film Thickness

In this example, the effect of thin film thickness on theone-dimensional periodic structures formed in response to parallelmagnetic fields was examined. The homogeneous Fe₃ O₄ magnetic fluid wassealed into glass cells with different cell depths, allowing the effectof film thickness to be investigated. FIG. 9 shows that the distancebetween particle chains was found to be proportional to the thickness ofthe thin film in the range of thickness from 10 μm to 2 μm when allother parameters were held constant.

EXAMPLE 8 Preparation of a Homogeneous MnFe₂ O₄ Magnetic FluidComposition

500 mls. of an 8 molar solution of NaOH was made by adding 160 g of NaOH(95% grade, Nihon Shiyaku Industries, Ltd.) to a sufficient amount ofde-ionized water to bring the final volume to 500 mls. A second solutionwas made by mixing 0.1 moles of MnSO₄.H₂ O (99% grade, Hayashi PureChemical Industries, Ltd.) and 0.2 moles of FeCl₃.6H₂ O (97% grade,Showa Chemicals, Inc.) in a sufficient volume of de-ionized water tobring the final volume to 600 mls. A glass stirring bar was used tocontinuously stir the second solution while a sufficient volume of NaOHwas added to raise the pH and maintain it between 10 and 11, as MnFe₂ O₄precipitated out of the solution. The addition of NaOH was completed inunder about 2 minutes and the pH value was maintained through the step.During this step, the temperature was held at 80° C. The precipitationprocess took about 60 minutes.

50 mls. oleic acid (Showa Chemicals, Inc.) was added to the solutioncontaining the MnFe₂ O₄ precipitate to coat the particles with oleicacid. At first, the pH value dropped substantially, and an additionalvolume of the NaOH solution was added to keep the pH at around 10 duringthe coating process. During this procedure, the temperature was raisedto and maintained at 95° C. The coating process took approximately 50minutes. At the end of this step, the solution separated into threephases. The upper phase was removed and discarded, and the middle andbottom phases were retained for use in the following step.

A volume of HCl (37%, Nihon Shiyaku Industries, Ltd.) was added to theretained solution sufficient to bring the pH down to about 1. This stepwas carried out at room temperature. The solution was stirred for about20 minutes, as magnetic particles coated with oleic acid began tocoagulate. At the end of the 20 minutes, the solution separated into twophases. The top phase, containing a black particulate suspension, wasremoved, and the entire acidification procedure was repeated as before.Again the top phase was removed and discarded. The black particulatesuspension was not observed in the top phase formed following the secondacidification step. In some syntheses, this step may have to be repeatedan additional one or two times, until the black particulate suspensionno longer is observed in the top phase, removing the top phase betweenrepetitions of the process.

After the acidification steps were completed, de-ionized water at 65° C.was added to the retained bottom phase in order to remove remaining HCland NaCl. The water was added as the material is stirred. A sufficientvolume of water was added to raise the pH of the material to around 5.The solid material was allowed to settle and the water was decanted.

The solid material was washed by adding 1.1 mls. of kerosene per gram ofsolid. This was stirred until the solid material was completelydispersed in the kerosene, and the solution was placed in a centrifugetube and centrifuged for 10 minutes at around 500×g. This and all othercentrifugation steps were carried out at room temperature. Aftercentrifugation, the sample had separated into two layers. The top layerwas a dark colored liquid containing salt residues and large particles,and the lower layer was a solid phase which contained magnetic particlescoated with oleic acid. The upper phase was removed, and the magneticparticles were dehydrated by suspending the material in ethyl alcohol,pelleting it by centrifugation for 15 minutes at around 2800×g, removingthe ethyl alcohol, and drying the magnetic particles in a 65° C. ovenfor 10 to 14 hours. Finally, the dried particles were dispersed intokerosene using a kerosene-to-particle ratio of 1.7 ml/g. This wascentrifuged again at around 2800×g for 10 minutes. The supernatant wasremoved from the test tube and placed in a 65° C. oven for approximately14 hours to drive off a portion of kerosene and thereby raise the MnFe₂O₄ concentration. The fluid was removed from the oven and aliquots ofthis homogeneous magnetic fluid were sealed into glass cells to formmagnetic fluid thin film.

An X-ray diffraction pattern for the MnFe₂ O₄ sample verified the singlephase fcc spinel structure of the sample. The lattice constant wasmeasured to be 8.45 Å. The magnetization of the sample was measuredusing a vibration sample magnetometer, and a saturation magnetizationvalue of 12.98 emu/g was measured for the homogeneous MnFe₂ O₄ magneticfluid. The volume fraction of the homogeneous magnetic fluid wascalculated as the ratio of the saturated magnetization of the magneticfluid to that of dry MnFe₂ O₄ powder. This ratio was 27.8%.

EXAMPLE 9 Two-dimensional Ordered Structures in MnFe₂ O₄ Thin Film as aFunction of Applied Field Strength

The laboratory setup shown in FIG. 2 was used to examine patternformation in a thin film of the homogeneous MnFe₂ O₄ magnetic fluid thinfilm synthesized in Example 8 in response to an externally appliedmagnetic field oriented perpendicularly to the plane of the film. Inthis example, the strength of the applied field was varied. FIG. 10shows images taken of a 6 μm thick magnetic fluid thin film using a CCDvideo camera that demonstrate the evolution of the two-dimensionalordered structure pattern from disorder quantum columns (FIG. 10a), toan ordered hexagonal structure (FIGS. 10b and 10c), and to a disorderedlabyrinthine pattern (FIG. 10d). These images demonstrate that thedistance between columns is proportional to the inverse of the fieldstrength in the range between two critical strengths, H_(h) and H_(l).

EXAMPLE 10 Two-dimensional Ordered Structures in MnFe₂ O₄ Thin Films asa Function of Film Thickness

In this example, the effect of film thickness on pattern formation wasexamined. The two-dimensional ordered structures in homogeneous MnFe₂ O₄magnetic fluid thin films with different thicknesses were investigatedusing the setup illustrated in FIG. 2. During this experiment, allparameters remain unchanged except the thickness of the film, which wasvaried from 10 μm to 2 μm by using glass sample cells having differentcell depths. FIG. 11 provides examples of images of thin films of thehomogeneous magnetic fluid synthesized in Example 8 taken by the CCDcamera using a constant field strength of 75 Oe in which atwo-dimensional hexagonal structure had formed in the films underinvestigation. These images indicate that the distance between columnsis roughly proportional to the thickness of the films over the range offilm thickness examined.

The results obtained in Examples 9 and 10 show that a two-dimensionalhexagonal structure forms in homogeneous MnFe₂ O₄ magnetic fluid thinfilms that are subjected to an externally applied perpendicular magneticfield. The distance between columns is closely related to the inverse ofthe field strength and is roughly proportional to film thickness, atleast over the range of thickness shown in Example 10. FIG. 12 plots thedistance between columns as a function of MnFe₂ O₄ film thickness andmagnetic field strength.

EXAMPLE 11 Two-dimensional Ordered Structures in MnFe₂ O₄ Films as aFunction of the Rate of Change of Field Strength

The effect of the rate of change of the magnetic field strength wasinvestigated, using rates of 5 Oe/s, 20 Oe/s, 50 Oe/s, and 100 Oe/s.FIG. 13 shows the relationship between column distance as a function offield strength using different rates of field strength change (dH/dt).The figure shows that as the rate is increased, the curves are displaceddownward and to the left. That is, as the rate of field strength changeincreases, the distance between the columns decreases.

EXAMPLE 12 Two-dimensional Ordered Structures in MnFe₂ O₄ Thin Films asa Function of Volume Fraction Ratio

MnFe₂ O₄ magnetic fluid samples with varying volume fraction ratiosbetween the magnetic particles and the carrier liquid were madeaccording to the method of Example 8, except in the final dispersingstep, the volume of kerosene added was altered to vary the volumefraction ratio of the fluid. FIG. 14 shows that a decrease in the volumefraction ratio produces a shift in the distance versus field strengthplots toward the upper right That is, holding all other parametersconstant, a decrease in the volume fraction ratio increases the distancebetween columns.

In the following two examples, the solenoid was replaced by Helmholtzcoils in the setup shown in FIG. 2. As a result, the orientation of themagnetic field applied to the thin film was parallel to the plane of thefilm.

EXAMPLE 13 One-dimensional Ordered Structures in MnFe₂ O₄ Thin Films asa Function of Applied Field Strength

In this example, the homogeneous MnFe₂ O₄ magnetic fluid thin film wassubjected to an externally applied magnetic field that was parallel tothe plane of the film. As the field was applied, the magnetic particlesin the film agglomerated and formed chains in the plane of the thin filmoriented along the field direction. These particle chains exist indifferent layers over the thickness of the film. When the field strengthwas increased, the chains became periodic and the distance between thechains decreased proportionately. FIG. 15 shows the effect of varyingthe field strength from 50 Oe to 300 Oe on the distance between theperiodic particle chains in the homogeneous MnFe₂ O₄ magnetic fluid thinfilm.

EXAMPLE 14 One-dimensional Ordered Structures as a Function of MnFe₂ O₄Film Thickness

In this example, the effect of thin film thickness on theone-dimensional periodic structures formed in response to parallelmagnetic fields was examined. The homogeneous MnFe₂ O₄ magnetic fluidwas sealed into glass cells with different cell depths, allowing theeffect of film thickness to be investigated. FIG. 16 shows that thedistance between particle chains was found to be proportional to thethickness of the thin film in the range of thickness from 10 μm to 2 μmwhen all other parameters were held constant.

EXAMPLE 15 First Type of Light Diffraction Color Display

When an applied perpendicular magnetic field reaches a critical valueH_(h), a two dimensional column array is formed in a homogeneousmagnetic fluid thin film. Diffraction phenomena occur as a parallelwhite light ray passes through the film, and constructive anddestructive interference occurs as the light rays reach the eyes of aviewer. FIG. 20 is a cross section drawing of arrays formed in ahomogeneous magnetic thin film illustrating the light diffractionconcept. In this Figure, d is the distance between columns in atwo-dimensional column array, θ is the angle formed between the incominglight ray and the direction perpendicular to the plane of the film, θ'is the angle formed between the diffracted rays and the directionperpendicular to the plane of the film, and N is the total number ofmagnetic particle columns diffracting the light. After diffraction, theintensity of the light, I, is: ##EQU2## where φ ##EQU3## and λ is thewavelength of light. The condition under which the light intensity, I,becomes maximum is the same as that under which the light becomesbrightest after diffraction through the film. This condition is:##EQU4## where κ is a non-negative integer.

The angle θ can be designed such that sin θ>>sin θ'. For a fixed angleθ, the color observed by the viewer will not change due to the limitedmovement of the viewer when the viewer is far away from the film.Meanwhile, the condition of κ=0 will never occur. The condition of κ=1is the most interesting and important one. Under this condition, d willbe related to λ by: d sin θ=λ.

If this wavelength, λ, is within the range of visible light, then thesame d also will allow only light with a wavelength of λ/κfor κ=2, 3 . .. to pass through the film. Fortunately, light with these wavelengthsare outside the visible spectrum. The reason for this is that thelongest wavelength of light visible to the human eye is about 0.7 μm,and so the wavelength of λ/2=0.35 μm. This wavelength is in theultraviolet region of the electromagnetic spectrum and therefore is notvisible to the human eye. Consequently, the viewer will only observe asingle wavelength of light.

Of course, there will be dispersion for the intensity, I. The degree ofthe dispersion δλ must satisfy the condition ##EQU5## In the case of atwo dimensional column array of a homogeneous magnetic fluid thin film,N is very large and depends on the area of the film. Thus, ##EQU6## isvery small. If the distance between columns, d, satisfies ##EQU7## apure monochromatic color will be observed. Fortunately, the distancebetween the columns in two dimensional column arrays of homogeneousmagnetic fluid prepared according to the methods of the presentinvention is on the order of several micrometers. Therefore, the arrayis capable of diffracting visible light to produce intensityinterference. Furthermore, because the distance d can be manipulated by,e.g., controlling the strength of the externally applied magnetic field,the rate of change of the magnetic field strength (dH/dt), the anglebetween the magnetic field and the film, the thickness of thehomogeneous magnetic fluid thin film, and/or its temperature, the colorof the light observed by the viewer can be changed at will.

A display constructed according to the methods of the present inventionwill comprise many pixels. Each pixel is made of a homogeneous magneticfluid thin film with an electronic circuit. The electronic circuit isused to drive the change of the column distance in individual pixels,resulting in a change of color of the outgoing light. FIG. 21 is aconceptual drawing of a pixel. As the distance, d, of the pixels in adisplay device are individually adjusted, the display will generate apolychromatic image.

Diffraction phenomena also will occur in a homogeneous magnetic fluidthin film under the influence of an externally applied parallel magneticfield, according to the diffraction principles set out by Bragg. As thefield is applied to the film, the magnetic particles agglomerate andform chains parallel to the plane of the thin film. The distance betweenchains can be controlled by changes in the field strength. When anincident white light beam forms an angle with the plane of the thinfilm, chains will reflect the beam. Since these chains are in differentlayers inside the film, the reflection of light by chains at differentlayers will interfere, resulting in a very sharp color. Here again, theexternally applied magnetic field can be adjusted to obtain the desiredcolors.

EXAMPLE 16 Monochromatic Light Diffraction Switch

As provided in Example 15, the homogeneous magnetic fluid thin film canbe used to create monochromatic light with wavelength λ from whitelight. Under the same conditions as described in Example 15; i.e.,sinθ>>sinθ' the diffraction occurs only when the column distance in thetwo dimensional column array of the homogeneous magnetic fluid thin filmsatisfies the condition of d sinθ=λ. That is, under a particularstrength of external magnetic field, a monochromatic color of light isdiffracted by the film and passes through it to reach the eyes of theviewer. By adjusting the field strength of the externally appliedperpendicular magnetic field, one should be able to close or open thelight switch. If there is a color dye covering the film, the desiredcolor will appear by opening the switch.

EXAMPLE 17 Tunable Wavelength Filter by Light Diffraction

This example also uses the concepts developed in Example 15. The columndistance of the hexagonal structure formed in the homogeneous magneticfluid thin film is adjustable and is around several micrometers. Asmentioned in Example 15, one can select any specific electromagneticwave with a wavelength on the order of the column spacing by adjustingthis spacing, d. The design of the homogeneous magnetic thin film andits electronic circuitry are similar to those illustrated in Example 8,except the area of the thin film may be substantially larger.

EXAMPLE 18 Second Type of Light Diffraction Color Display

The idea of the second type of light diffraction color display is acombination of the technologies used in Example 15 and Example 16. Thisdisplay consists of a large number of pixels. Each pixel includes threemonochromatic light diffraction switches placed adjacent to each other.Each switch is made of a homogeneous magnetic fluid thin film with anaccompanying electronic circuit for controlling the distance, d. Thelight sources for the three switches are red, green, and blue,respectively. The switches are set to allow only the passage of red,green, or blue light, individually.

By properly adjusting the current in the control circuits, one is ableto turn the monochromatic light switch on or off, and hence allow noneor one of these three colors to pass through its switch. Therefore eachpixel of the display will show either black, red, green, blue, or anycombination of these three colors. When the currents of the switchescomprising the pixels are adjusted individually, the display willgenerate a colorful RGB (red, green, blue) picture.

EXAMPLE 19 Light Double Refraction Color Display

This example is an application of the use of homogeneous magnetic fluidthin films under an external magnetic field oriented parallel to theplane of the thin film. Under the applied field, the magnetic particlesagglomerate and form chains in the plane of the film. These chains existat different layers over the thickness of the film. The magnetic fluidinside the thin film becomes an anisotropic medium due to thedirectional arrangements of the particle chains.

The light refraction index n.sub.∥, along the direction of the chainswill be different from the light refraction index, n.sub.⊥, along thedirection perpendicular to the chains. Thus, after traveling a distance,s inside the magnetic fluid, in which s is the thickness of the film,the plane of polarization of a light wave with the electric fieldparallel to the direction of the chains will be different from that withthe electric field perpendicular to the direction of the chains.Denoting these field strengths by E.sub.∥ and E.sub.⊥, they are:##EQU8## where ##EQU9## ω is the frequency of the electromagnetic wave,t is time, and c is the speed of light. These electromagnetic wavesinterfere due to the different values of n.sub.∥ and n.sub.⊥. Thisexample is an application of control of the interference of two lightwaves by adjusting the strength of the externally applied magneticfield, resulting in changes in the difference between n.sub.∥ andn.sub.⊥. FIG. 22 illustrates the invention embodied in this example.

In FIG. 22, two polarizers, with their polar axes perpendicular to eachother, cover both sides of a homogeneous magnetic fluid thin film. Theexternally applied magnetic field is chosen such that its fielddirection forms a 45° angle with the polar axes of both polarizers. Whenlight impinges on the polarizer, only light parallel to the direction ofthe polar axis of the polarizer will be transmitted. Since these twopolarizers are perpendicular to each other, the light which passesthrough the first polarizer can not pass through the second polarizer.However, when there is an anisotropic medium between the two polarizers,the electric field of the incoming light is rotated. Thus some of thelight will be able to pass through the second polarizer.

In this example, an external magnetic field is applied parallel to thefilm, and the magnetic fluid inside the film becomes anisotropic. As aresult of the difference created between n.sub.∥ and n.sub.⊥, the planeof polarization of the light will be rotated as it passes through thefilm. Therefore, some light will be able to pass through the secondpolarizer and reach the eyes of the viewer. Practically, the intensityof light that passes through both polarizers and the homogeneousmagnetic fluid thin film is proportional to ##EQU10## The condition forthe maximum intensity is ##EQU11## in which κ is a non-negative integer.In this case, s, the thickness of the film can not be changed bychanging the external magnetic field. However, the value of (n.sub.∥-n.sub.⊥) can be changed by changing, e.g., the strength of the externalfield. Thus, one is able to obtain light with the desired wavelength by,e.g., adjusting the strength of the external magnetic field. The pixeland the electronic circuit that drives the magnetic field are similar tothose shown in Example 15, with the only difference being that, in thisexample, the magnetic field is parallel to the plane of the homogeneousmagnetic fluid film.

The above examples are provided to illustrate the invention but not tolimit its scope. Other variants of the invention will be readilyapparent to one of ordinary skill in the art and encompassed by theappended claims. All publications, patents, and patent applicationscited herein are hereby incorporated by reference.

What is claimed is:
 1. A magnetic fluid composition, consistingessentially of: MnFe₂ O₄ particles coated with a surfactant anddispersed in a continuous phase carrier liquid, wherein the compositionis capable of forming a crystalline array when exposed to an externalmagnetic field.
 2. A method of synthesizing a magnetic fluid compositioncapable of forming a crystalline array when exposed to an externalmagnetic field, comprising the steps of:(a) precipitating MnFe₂ O₄ froman aqueous solution consisting of FeCl₃ and MnSO₄ solution by adding asufficient amount of an hydroxide-containing base to raise the pH of thesolution to and maintain it at between 10 and 11, wherein the timeelapsed from the start of the alkali addition and the attainment of a pHvalue of 11 is less than or equal to 2 minutes; (b) coating the MnFe₂ O₄particles with a surfactant; (c) coagulating the coated MnFe₂ O₄particles by adding a sufficient amount of an acid to reduce the pH ofthe liquid containing the coated particles to pH 1 or below; (d)recovering the coated MnFe₂ O₄ particles and suspending them in a volumeof water sufficient to raise the pH of the suspension to between 4.7 and5.0; (e) recovering the coated MnFe₂ O₄ particles from the aqueoussuspension; (f) washing the recovered coated MnFe₂ O₄ particles in acarrier liquid; (g) dehydrating the coated MnFe₂ O₄ particles; and (h)suspending the coated MnFe₂ O₄ particles in the carrier liquid.
 3. Themethod of claim 2, wherein the base is NaOH, the surfactant is oleicacid, and the carrier is kerosene.
 4. The method of claim 2, wherein thedehydrating step comprises the steps of:(a) suspending the coated MnFe₂O₄ particles in ethyl alcohol; (b) recovering the coated MnFe₂ O₄particles from the ethyl alcohol; and (c) drying the recoveredparticles.